TreeTable collapseAll does not work

Hello All, I am developing treeTable using Odata. It is working perfectly. It looks as following:

The problem is, it doesn't collapse to Level 0 when I use

oTable.collapseAll();

even oTable.expandToLevel(0); doesn't work.

Is there anything I am doing wrong? Please help me out. I have spent too much time on this.

I am using same concept as shown in "SAP UI5 Explored" website.

Thank you.